The invention relates to constant-speed synchrophasing of the propellers of an aircraft having several propulsion units, each propulsion unit comprising an engine or a turbine and a propeller driven by the engine or the turbine.
In such aircraft, it is important for the comfort of the passengers to reduce cabin noise as much as possible. When they pass in the direction of the fuselage, the acoustic waves emitted by the propeller blades of the aircraft constitute a major source of noise. A significant noise reduction can be achieved by suitably distributing the acoustic wave impacts on the fuselage over time, in particular to avoid simultaneous impacts. To this end, the relative angular positions of the propellers of the different propulsion units are adjusted. This is what is meant by synchrophasing.
Synchrophasing the propellers of an aircraft to reduce cabin noise is well-known. The document U.S. Pat. No. 5,551,649 is representative, inter alia, of the prior art in this field.
Synchrophasing is implemented by maintaining the relative angular positions between propellers at predetermined values which may have been obtained during test flights. The relative angular position values can be expressed in the form of phase displacements between propellers, the phase of a propeller being then defined as the angular difference between the actual angular position of the propeller and a reference angular position common to all the propellers. The phase can be expressed in degrees, with one complete propeller rotation representing 360°.
Synchrophasing is performed at constant speed, that is to say, for the same constant rotational speed of the propellers. Several constant speeds can be adopted corresponding to different flight phases (cruising, takeoff, etc.) or to different flight conditions (wind, etc.) of the aircraft, the number thereof generally being limited. Different desirable phase displacement values can be determined for different respective constant speeds.
When the aircraft is operating, for a given constant speed, the objective is to maintain the actual values of phase displacement between the propellers equal to the determined desirable values.
To this end, provision is made in each propulsion unit for a sensor which delivers a signal indicating the angular position of the propeller. Such a sensor can, for example, comprise a magnetic element associated with a blade of the propeller and which passes across a fixed detector at each revolution.
In the prior art, one of the propulsion units of the aircraft is chosen as the “master” propulsion unit providing a reference of the angular position of the propeller. In the other or each of the other “slave” propulsion unit(s), the signal produced by the angular position sensor is compared with that provided by the master propulsion unit sensor in order to determine an actual phase displacement value. The latter is compared with the determined desirable value and, where it is different, an instantaneous variation of the propeller speed is applied to substantially cancel out this difference. The speed variation can be produced by acting on a propeller pitch control device interposed between the engine or the turbine and the propeller, in which case the latter is of the variable-pitch type.
One drawback of known architectures of this type lies in the existence of a point-to-point connection between the master propulsion unit and the other or each of the other slave propulsion unit(s) to transmit the signal from the angular position sensor of the propeller of the master propulsion unit. Such an electrical connection requires a particular cabling arrangement which is exposed to the risk of collapse and which, together with its insulation and fixing means, represents a not inconsiderable weight.
Furthermore, the existence of such point-to-point connections between the propulsion units creates a risk of propagation of failure and means that the different propulsion units cannot entirely be treated as independent identical entities, which can result in additional constraints for certification.